This invention relates generally to ultrasonic surgical apparatus. More particularly, this invention relates to an improved method and apparatus for generating profiled pulses of ultrasonic frequency vibratory energy at a distal surface of an ultrasonic applicator of an ultrasonic surgical instrument for application to tissues of a patient with specific relationships between a magnitude of the pulse of ultrasonic frequency vibratory energy and a duration of the pulse of ultrasonic frequency vibratory energy so that the ultrasonic applicator can be driven to vibratory amplitudes previously not achievable and a more expedient surgical effect obtained.
Ultrasonic surgical devices typically operate at frequencies between 20 kHz and 60 kHz and have application in many surgical specialties including neurosurgery, general surgery, and ophthalmic surgery. In general it is known that ultrasonic surgical devices generate ultrasonic frequency vibratory energy that is applied to an ultrasonic applicator that vibrates longitudinally and which contacts the tissues of a patient. The ultrasonic surgical device may, among other surgical effects, cut, fragment, and/or coagulate the contacted tissues of the patient.
Ultrasonic surgical devices are constrained in their ability to generate ultrasonic frequency vibratory energy due to limits imposed by machining tolerances and by limits inherent in the physical characteristics of the materials used to fabricate the devices. For example, titanium alloys are often used for fabrication of the ultrasonic applicator that is used to contact the tissues of a patient. Titanium alloys have inherent fatigue strength and stress limitations that cannot be exceeded or the ultrasonic applicator will crack. As a further example, the ultrasonic motor that converts supplied electrical power to ultrasonic frequency vibratory energy may be fabricated from piezoelectric ceramics. Piezoelectric ceramics have inherent limitations on their ability to efficiently convert electrical energy to vibratory energy, including limits on applied voltage so that the ceramic elements do not loose their piezoelectric properties.
However, a phenomenon referred to in this disclosure as `mode coupling`, is most often responsible for establishing the upper performance bound of an ultrasonic surgical device. Mode coupling occurs when the vibratory amplitude of an ultrasonic applicator of an ultrasonic surgical device is increased to such a level that the ultrasonic frequency vibratory energy at the desired resonant frequency is coupled to other modes of vibration, referred to herein as `parasitic modes`. The parasitic modes of vibration may be at lower frequencies, near-by frequencies, or higher frequencies, depending of the design of the system. The parasitic modes of vibration may be longitudinal modes or they may be transverse modes, or they may be more complicated coupled modes. Mode coupling is especially troublesome when the ultrasonic applicator is an elongate probe or catheter with a length greater than one wavelength at the resonant frequency of the particular ultrasonic surgical device. Mode coupling may occur for ultrasonic applicators shorter than one wavelength and may also occur for ultrasonic applicators that are not shaped like an elongate probe, for example, flat or convex radiating surfaces.
The most common type of mode coupling encountered for ultrasonic surgical devices is the stimulation of a lower or near-by frequency transverse mode so that the ultrasonic applicator vibrates in the desired longitudinal vibratory mode and an undesired transverse vibratory mode simultaneously. This type of coupled vibration can easily cause stresses in the ultrasonic applicator material sufficient to break the ultrasonic applicator.
Ultrasonic surgical devices that operate at high vibratory amplitudes also generate undesirable heat, primarily in the ultrasonic motor, but also in the material of the ultrasonic applicator due to internal friction and other losses as the ultrasonic applicator vibrates. If the ultrasonic motor becomes too hot during a typical procedure then active cooling, such as forced air or water cooling, of the ultrasonic motor is required, making the ultrasonic surgical handpiece more expensive and more cumbersome due to the additional supply lines. If the ultrasonic applicator becomes hot then the tissues of a patient may be unnecessarily burned.
Mode coupling and heat generation have placed fundamental limits on the performance of ultrasonic surgical systems. What has been discovered, and is disclosed herein, is an ultrasonic surgical apparatus and method for generating profiled pulses of ultrasonic frequency vibratory energy such that mode coupling is suppressed or eliminated so that the ultrasonic applicator can be driven to desired vibratory amplitudes which were previously unobtainable, thus increasing the expediency of a surgical procedure. Further, because the expediency of the surgical procedure is increased, the effective dose of ultrasonic frequency vibratory energy delivered to the tissues of a patient is minimized. Still further, because the ultrasonic applicator is driven to high vibratory amplitudes for only short periods of time, internal heating of the ultrasonic applicator is reduced, as is the electrical power consumed by the ultrasonic motor.
The use of switchable or pulsed vibratable tools is disclosed in patents. U.S. Pat. No. 4,614,178 to Harlt has a dose meter and a control circuit for switching the mode of operation in an ultrasonic therapeutic apparatus. A detector circuit is used to monitor the output to a treatment head so that a time measurement of the duration of the treatment can be switched between an enabled or disabled state. This therapeutic, not surgical, device is intended to deliver heat to the tissues of a patient and the switch between states of operation is used to ensure that a proper dose of heat is delivered to the patient.
U.S. Pat. No. 3,980,906 to Kuris has a driving circuit for producing bursts of ultrasonic frequency oscillations between 10 kHz and 1,000 kHz at repeated sonic intervals in the range of 10 Hz to 1,000 Hz, the repeated sonic intervals of ultrasonic frequency oscillation applied to ultrasonic instruments such as toothbrushes and razors. This patent uses bursts of ultrasonic energy to reduce sliding friction for smoother motion when shaving and to provide a satisfactory tactile sense of operation to a user. Each burst of ultrasonic mechanical vibration lasts for 1/2 of the sonic interval, resulting in on-off intervals of equal duration.
U.S. Pat. No. 4,343,111 to Inoue has an ultrasonic machining method wherein the vibratory energy is intermittently interrupted to create a series of time-spaced bursts of vibratory oscillation and the frequency or amplitude of the vibration is modified during each of the bursts. This patent uses of bursts of ultrasonic energy to reduce surface roughness of machined metal parts and to machine irregular contours into metal pieces.
U.S. Pat. No. 3,673,475 to Britton has a drive circuit for generating pulses that are applied to a dental impact tool with a reciprocating armature. This patent discloses a drive circuit to generate pulses to `pull-back` and then `drive` an armature, a technique that is not applicable to ultrasonic frequency vibratable tools.
None of the aforementioned patents teaches the use of profiled pulses of ultrasonic frequency vibratory energy for a surgical effect on tissues of a patient, none addresses using profiled pulses of ultrasonic frequency vibratory energy to suppress or eliminate the phenomenon described herein as mode coupling, and none suggests using profiled pulses of ultrasonic frequency vibratory energy to minimize internal heating in the ultrasonic applicator and the ultrasonic motor. The patents do not disclose any benefits due to relationships between the magnitude of the pulses of ultrasonic frequency vibratory energy and the duration of the pulses of ultrasonic frequency vibratory energy.
U.S. Pat. No. 4,827,911 to Broadwin has an ultrasonic surgical handpiece with a switching means for automatically and repeatedly switching the amplitude of ultrasonic vibration between a constant working high amplitude and a constant standby low amplitude, both used in combination with aspiration and irrigation, for enhanced fragmentation and improved surgical control. The invention works by interrupting continuous vibratory operation with on-off duty cycles, with suitable on-times for first, second, third, and fourth modes given as 50 milliseconds, 100 milliseconds, 150 milliseconds, and 200 milliseconds, respectively. The continuous vibratory operation is interrupted with a repetition rate of at least 30 Hz so that the operator does not distractedly sense the operation at low amplitude.
The Broadwin patent does not address or appreciate using profiled pulses of ultrasonic frequency vibratory energy to suppress or eliminate the phenomenon described herein as mode coupling, it does not address using profiled pulses of ultrasonic frequency vibratory energy to reduce heating in the ultrasonic applicator and the ultrasonic motor, nor does it disclose any benefits due to relationships between the magnitude of the pulses of ultrasonic frequency vibratory energy and the duration of the pulses of ultrasonic frequency vibratory energy.